There are two main sorts of grey gold alloys on the market: alloys in which the whitening metal for the gold is nickel, and those where this metal is palladium. However, alloys comprising nickel are used less and less due to their allergenic properties on contact with the skin, which precludes their use in external timepiece components. Consequently, palladium alloys are used for this function.
Grey gold alloys intended to be used in the fields of horology and jewelry must satisfy two constraints relating first to their brightness/whiteness and secondly to their capacity for deformation. They must therefore have pure white colour and brightness and excellent ductility and resistance to corrosion. More specifically, the desired grey gold alloys must have values in the L*a*b colour space (CIE 1976) such that L>80, a*<1.5 and b*<7, and preferably b*<6 and preferentially b*<5, and a Vickers hardness comprised between 140 HV and 225 HV, the lowest values being the most favourable for deformation.
As the whitening effect of palladium is less than that of nickel, these alloys necessarily have a high palladium content, which limits their mechanical properties. Further, rhodium plating is often used to improve the colour and reflectivity of the alloys, in order to enhance the brightness of the jewels when the alloys are used in settings.
This rhodium plating is a major long-term drawback because the rhodium plating layer, on the order of 1 to 5 microns, always eventually wears away. Consequently, the after-sales service is faced with an expensive re-plating operation, due to the need to conceal the difference in colour between the alloy and the rhodium improvement layer.
These colours can be compared through several references mentioned below.
EP Patent 1010768 relates to 18 carat grey gold alloys having a palladium content comprised between 12 and 14%, and also comprising copper, which gives colour values in the L*a*b colour space such that 1.8<a*<2.3 and 7<b*<10.
EP Patent 1227166 relates to 18 carat, palladium-free, grey gold alloys comprising copper and manganese, which gives colour values in the L*a*b colour space such that 2.6<a*<6 and 10<b*<13.
EP Patent 1245688 relates to 18 carat grey gold alloys having a palladium content comprised between 5 and 7%, also comprising copper and silver, which gives colour values in the L*a*b colour space such that 1.5<a*<4.5 and 10.5<b*<15.2.
The a* and b* colour values of the alloys presented in these three Patents are too high to claim that surface improvement by rhodium plating is unnecessary.
The Patent Application published under the number EP2546371A1 relates to 18 carat grey gold alloys having a palladium content comprised between 2 and 12% and a chromium content comprised between 13 and 23%, which gives colour values in the L*a*b colour space such that 0.25<a*<0.7 and 3<b*<4.2.
EP Patent No 2427582B1 relates to 18 carat grey gold alloys having a palladium content comprised between 18 and 24% and comprising between 1 and 6% of various elements comprising Zr, Nb or Mn, which gives colour values in the L*a*b colour space such that 1.1<a*<1.5 and 4.5<b*<5.7.
The alloys described in the latter two Patent Applications have sufficient a* and b* colour values to assert that surface improvement by rhodium plating is unnecessary. However, the hardness of these alloys—370 HV and 276 HV—is too high.
Tables 1 and 2 summarize the situation of the alloys previously disclosed in the prior art
TABLE 1AuPdAgGaFeCuCrMnZrNbInZnSiHVA7511303.510115145B75117950191165C751701012940188D7517030149370E751190192020276F7512102019184
TABLE 2La*b*A80.02.17.7B86.33.512.9C80.63.212.2D80.90.32.6E81.21.03.9F81.41.14.3EP Patent Application 3070182 A1 relates to a grey gold alloy which is nickel-free, cobalt-free, iron-free, silver-free, copper-free, zirconium-free, niobium-free, chromium-free and manganese-free, comprising, expressed in weight percent, from 75.0 to 76.5% of Au, from 15 to 23% of Pd, from 0.5 to 5% of Rh, from 0 to 7% of Pt, and from 0 to 5% of at least one of the alloying elements Ir, Ru, Ti, In, Ga, B and Re, the respective percentages of all the elements of the alloy adding up to 100%.Due to its precious metal content, this alloy can only be exclusively used for the manufacture of fine jewelry.